= Calibrated radiant-flux measurement
An <optical power meter> makes the definition operational: collect a specified beam on a calibrated <photodiode>, measure its <electric current>, and infer incident <radiant flux> using <spectral responsivity>. In the manufacturer-authored <Model 1830-R Series Optical Power Meters User's Manual by Newport Corporation>, §5.5.1 specifies a concrete sequence: connect the detector and its calibration module, select watt mode and the source <wavelength>, block the beam and press ZERO, then unblock it and read the display. The stored background includes <dark current> and ambient <light> present during zeroing. For narrowband radiation,
$$
\Phi_{\rm indicated}=\frac{i_{\rm illuminated}-i_{\rm zero}}{R(\lambda)}.
$$
The <electric current> difference divided by <spectral responsivity> has units $\mathrm A/(\mathrm{A\,W^{-1}})=\mathrm W$. For a stipulated $R=0.40\,\mathrm{A\,W^{-1}}$ and difference $0.20\,\mathrm{mA}$, the inferred <radiant flux> is $0.50\,\mathrm{mW}$; these numbers illustrate the inference and are not reported measurements. The entire beam must hit the active <area>, and §5.6 calls for correct <wavelength>, background subtraction, and operation below detector saturation. If an <optical attenuator> is fitted, §5.4 requires the calibrated detector–attenuator pair and the matching meter setting.
The calibration itself must connect <electric current> to independently known incident <power>. NIST's <Spectroradiometric Detector Measurements: Ultraviolet, Visible, and Near-Infrared Detectors for Spectral Power by Thomas C. Larason and others> describes this chain. A <cryogenic radiometer>, an <electrical substitution radiometer>, compares <temperature> changes from absorbed <light> with those from known <Joule heating> (§5.2). Transfer and working detectors carry that <watt> scale to a spectral comparator (§5.1). A source, <monochromator>, focusing optics, shutter, <beam splitter>, and monitor <photodiode> illuminate a standard and test detector in turn; calibrated current-to-voltage electronics and voltmeters provide the readout (§§3.2.2–3.2.4, 4.3.1). At each <wavelength>, subtract blocked-beam signals and normalize each detector signal by the simultaneously sampled monitor signal. Comparison with the standard yields the test detector's <spectral responsivity>. NIST uses two working standards and alignment normal to the beam (§6.1.1).
Controls distinguish a smaller collected <radiant flux> from a faulty conversion: block and unblock the beam to check the baseline; check that its footprint fits the active <area>; verify the calibrated <optical attenuator> configuration; and test proportional response within the specified operating range. Source drift faster than monitor sampling is not removed. NIST discusses finite spectral bandwidth, stray <light>, <temperature>, and spatial response variations (§§3.1.1, 6.2, 7.2). A broadband <optical spectrum> requires the weighted <integral> in <spectral radiant flux>, rather than dividing by an arbitrary single <spectral responsivity>. These limitations explain why the meter's displayed <watts> need a stated collection boundary, spectral assumptions, and calibration conditions.
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